The present invention relates generally to the field of photochromic glass and methods of making the same. More specifically, the present invention relates to photochromic glass. In especially preferred forms, the present invention is embodied in photochromic float glass (that is, glass made by a float process which exhibits photochromic properties).
The formation of flat glass by the float method (i.e., so-called xe2x80x9cfloat glassxe2x80x9d) is well known as evidenced, for example, by U.S. Pat. No. 3,843,346 to Edge et al (the entire content of which is expressly incorporated hereinto by reference). Generally, the float glass process includes floating a molten glass ribbon on a bath of molten metal (typically, molten tin or tin alloy). The float glass ribbon starts to cool and its thickness is established while on the molten metal bath. The ribbon is thereafter lifted off of the tin bath and conveyed into an annealing zone where it is controllably cooled to a temperature below its strain point. The annealed float glass sheet may then be subjected to further downstream cold and/or hot processes, such as, for example, cutting and final shaping.
Glass having photochromic properties is also well known as evidenced, for example, by U.S. Pat. No. 3,208,860 to Armistead et al (the entire content of which is expressly incorporated hereinto by reference). In essence, known photochromic glass are based principally on silver-halide inorganic glass chemistry which allows the glass to reversibly darken under exposure to actinic (ultraviolet) radiation. The increased darkness of the photochromic glass will then fade to the normal optical transmittance when exposure to the actinic radiation ceases.
Photochromisim in such glass is generally developed as a result of the formation of a micro-crystalline phase of silver halides in the glasses (typically after subjecting the glass to a thermal post-treatment). The formation of such micro-crystalline phase of silver halides in the glass is only possible in glass matrices prone to liquationxe2x80x94notably, alkali-borosilicate glasses. Thus, photochromic glass most typically are formed of an alkali-alumino-borosilicate glass-forming system with silver chloride and bromide.
The conventional wisdom in the art is that the float process cannot be employed to produce glass with photochromic properties. Specifically, the environment of the float glass process (for example, the molten tin or tin alloy and/or atmosphere within the float glass furnace) is chemically hostile to many of the compositional constituents needed to impart active photochromism to the resulting glass product. Thus, for example, such constituents either in the glass melt, the furnace atmosphere and/or in the tin or tin alloy bath on which the melt is floated may conflict in terms of undesirable reduction/oxidation reactions, formation of clusters, volatility of one component in the presence of others, discoloration or extreme coloration and the like.
Large sheets of photochromic glass therefore, while capable of being produced using a drawn sheet glass process (see, for example, U.S. Pat. No. 4,358,542 to Hares et al, the entire content of which is incorporated expressly hereinto by reference), are traditionally thought to be incapable of being produced by a float process. It would therefore be highly desirable if float glass exhibiting active photochromic properties could be provided. It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in photochromic float glass and methods of making the same. In especially preferred forms, the present invention is embodied in float glass having a non-photochromic glass substrate layer and a photochromic layer fused onto the substrate layer. During production, layers of the photochromic and non-photochromic glass are brought into contact with under conditions which fuse the layers one to another.
In one exemplary method for making the photochromic float glass of the present invention, a molten ribbon of non-photochromic glass is floated onto a molten metal (e.g., tin or tin alloy bath) within a float glass furnace. A second molten ribbon formed of a photochromic glass composition is then laid over the non-photochromic glass ribbon. The two glass ribbons thus fuse at their interface so as to achieve a unitary glass sheet comprised of a non-photochromic glass layer on the metal side of the sheet and a photochromic glass layer on the atmosphere side of the sheet. Alternatively, a molten ribbon of the photochromic glass may be brought into contact with a surface of a solid, but temperature-elevated, pre-made non-photochromic float glass layer so as to fuse the two layers one to the other.
The sheet of float glass can the be processed according to conventional practices such as annealing, heat treatment to develop and/or enhance/correct the photochromic properties of the glass, cutting, sizing and/or shaping.
As can be appreciated, the non-photochromic glass layer forms a barrier of sorts which protects the photochromic glass composition from the deleterious effects of the molten metal bath employed in conventional float glass processing. Moreover, according to the novel composition of the glass which ultimately forms the photochromic glass and/or the atmosphere within the float glass furnace are controlled so as to achieve the desired results. Notably, the photochromic glass composition employed in the present invention has a relatively high iron content (expressed as Fe2O3) which has traditionally been thought to impart a negative effect on the photochromic properties of a glass, decreasing its sensitivity to actinic radiation. Additionally (or alternatively), hydrogen is present in relatively small, but meaningful, amounts in the atmosphere of the float glass tin or tin alloy bath so as to minimize (if not eliminate entirely) undesirable surface reduction, and thus its attendant discoloration, of the photochromic glass composition exposed to such atmosphere.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.